At least 1 million men suffer from prostate cancer and it's estimated that the disease will strike one in six U.S. men between the ages of 60 and 80. There are more than 300,000 new cases of prostate cancer diagnosed each year. Prostate cancer will affect one in six men in the United States, and the mortality from the disease is second only to lung cancer. An estimated $2 billion is currently spent worldwide on surgical, radiation, drug therapy and minimally invasive treatments, $1 billion of the spending in the U.S. There is presently no effective therapy for relapsing, metastatic, androgen-independent prostate cancer. New agents that will enable rapid visualization of prostate cancer and specific targeting to allow radiotherapy present are needed.
N-acetylated alpha-linked acidic dipeptidase (NAALADase), also known as glutamate carboxypeptidase II (GCPII) is a neuropeptidase which cleaves N-acetylaspartyl-glutamate (NAAG) into N-acetylaspartate and glutamate in the nervous system, see below, depicting hydrolytic cleavage of NAAG by NAALDase through the tetrahedral intermediate. The enzyme is a type II protein of the co-catalytic class of metallopeptidases, containing two zinc atoms in the active site.

Independent of its characterization in the nervous system, one form of NAALADase was shown to be expressed at high levels in human prostatic adenocarcinomas and was designated the prostate-specific membrane antigen (PSMA). The NAALADase/PSMA gene is known to produce multiple mRNA splice forms and based on previous immunohistochemical evidence, it has been assumed that the human brain and prostate expressed different isoforms of the enzyme.
Human prostate-specific membrane antigen (PSMA), also known as folate hydrolase I (FOLH1), is a trans-membrane, 750 amino acid type II glycoprotein which is primarily expressed in normal human prostate epithelium but is upregulated in prostate cancer, including metastatic disease. PSMA is a unique exopeptidase with reactivity toward poly-gamma-glutamated folates, capable of sequentially removing the poly-gamma-glutamyl termini. Since PSMA is expressed by virtually all prostate cancers and its expression is further increased in poorly differentiated, metastatic and hormone-refractory carcinomas, it is a very attractive target for prostate imaging and therapy. Developing ligands that interact with PSMA and carry appropriate radionuclides may provide a promising and novel targeting option for the detection, treatment and management of prostate cancer.
The radio-immunoconjugate form of the anti-PSMA monoclonal antibody (mAb) 7E11, known as the PROSTASCINT scan, is currently being used to diagnose prostate cancer metastasis and recurrence. Early promising results from various Phase I and II trials have utilized PSMA as a therapeutic target. PROSTASCINT targets the intracellular domain of PSMA and is thought to bind mostly necrotic portions of prostate tumor.14 More recently, monoclonal antibodies have been developed that bind to the extracellular domain of PSMA and have been radiolabeled and shown to accumulate in PSMA-positive prostate tumor models in animals.
While monoclonal antibodies hold promise for tumor detection and therapy, there have been limited clinical successes outside of lymphoma because of their low permeability in solid tumors. Low molecular weight mimetics, with higher permeability in solid tumors will have a definite advantage in obtaining high percent per gram and a high percentage of specific binding.